In EGPRS systems (including “derivatives” such as EGPRS2), the use of so called MIMO (Multiple Input, Multiple Output) technology has now been introduced. In systems that support MIMO, multiple data streams (also sometimes referred to as “layers”) are transmitted simultaneously between a transmitting party and a receiving party. Usually, a plurality of transmit antennas are used, and each data stream is transmitted from one or more of the transmit antennas. Each data stream can also be given a so called weight factor, so called pre-coding of the data streams. Often, the pre-coding is based on a so called codebook, i.e. a list of the pre-coding weights used.
Pre-coding of the data streams will give the best SNR (Signal to Noise Ratio) in the receiver, but a MIMO transmitter will usually make some non-precoded transmissions as well, since non-precoded transmissions allow the receiver to estimate an optimal pre-coder or optimal pre-coding weights, which the transmitter can then use for future transmissions.
Thus, in a system which supports EGPRS MIMO which can use precoding it is desirable to use as many precoded transmissions as possible in order to obtain the highest SNR possible, while it at the same time is desirable to use as many non-precoded transmissions as possible in order to obtain an optimal estimate of the pre-coder. As can be seen, these two desires conflict.
An EGPRS system can use transmissions of dummy blocks (i.e. blocks which contain no payload) over a Temporary Block Flow, a TBF, in order to keep the TBF alive. In addition, in EGPRS, so called preemptive retransmissions of RLC data blocks can be used when a transmitter has transmitted unacknowledged data and there is nothing else to schedule for transmission. In this case, transmission of a payload is repeated, even though it is not known whether or not a previous transmission of the same payload was successfully received or not.